The present invention generally relates to the field of radio frequency signal monitoring and measurement, and more particularly, is directed to a device for monitoring various parameters of a radio signal such as level of modulation.
From time to time, the FCC establishes requirements with respect to the percentage modulation of radio frequency transmitters. Current regulations require that transmitters in the maritime service generally have a peak modulation between 75 and 100 percent. In land based services, such as point-to-point microwave transmissions, peak modulation must be maintained between 70 and 100 percent.
With respect to AM, FM and TV signals used in domestic fix public services, the percentage of modulation must be maintained at as high a level as possible, consistent with good quality of transmission and good broadcast service. As a general rule, the modulation level should not be less than 85 percent on peaks of frequent recurrence. A lower modulation level is permitted when required to avoid objectional loudness or maintain the dynamic range of the program material. With respect to maximum modulation levels, AM stations may not exceed 100 percent modulation on negative peaks of frequent recurrence or 125 percent modulation on positive peaks of frequent recurrence. FM stations are limited to 100 percent modulation on peaks of frequent recurrence referenced to 75 KHz deviation. FM stations which provide subsidiary communication services using subcarriers concurrently with their normal program broadcast are permitted higher levels of modulation. Total peak modulation may be increased by 0.5 percent for each 1.0 percent subcarrier injection modulation to a maximum of 110 percent carrier modulation at 82.5 KHz peak deviation. The FCC regulations also require that with respect to stereo FM signals, the 19 KHz pilot signal must be transmitted at a level not in excess of 10 percent of full modulation and not to be less than 8 percent of full modulation. In the transmission of television signals, modulation of the aural carrier usually is limited to 100 percent of frequent recurrence.
Many stations, in an attempt to comply with the FCC regulations regarding modulation, resort to various compression or limiting amplifiers in an attempt to assure they do not exceed the modulation limits. Over modulation not only subjects the station to action by the FCC for violating its regulations, but may also cause crosstalk in an adjacent FM station. However, the use of such limiting devices may cause the percent modulation to be below the required minimum, which also could lead to FCC action as well as degrade the quality of the transmission. Moreover, for commercial reasons many stations wish to modulate their signals to the highest level permitted.
In many cases, even with the use of limited modulation to the transmitter, stations can be either consistently over or under modulating the carrier. To a large extent, this difficulty is due to the lack of a good dependable monitoring system capable of making available an instant, accurate means with which to check percentage modulation transmission of various components of the transmitted signal while maintaining regular station programming.
Prior art monitoring systems have frequently utilized meter indicators to give a visual indication of the amount of modulation of the carrier. The difficulty with such systems is that they require frequent calibration to correct for any change in the ballistic considerations of the meter. In addition, meter readings always are subject to some degree of interpretation and parallax.